1. Field of the Invention
This invention relates to an image forming apparatus and, more particularly, to an image forming apparatus using, e.g., an electrophotographic method for forming images by transferring toner images formed on an image carrier to a transfer material.
2. Description of Related Art
Electrophotographic multicolor printers of an intermediate transfer type, conventionally form images by overlapping toner images formed on respective photosensitive drums corresponding to respective colors in, e.g., black (K), yellow (Y), magenta (M), and cyan (C) to an image carrier (or an intermediate transfer body) upon transferring the toner images as a primary transfer and subsequently by transferring the toner images carried on the image carrier onto a recording medium again as a secondary transfer.
Japanese Patent Application Publication No. 2012-053085 discloses arts on electrophotographic multicolor printers of the intermediate transfer type. In such a prior art electrophotographic multicolor printer of the intermediate transfer type, respective primary transfer rollers are disposed in contact with respective photosensitive drums corresponding to respective colors in, e.g., black (K), yellow (Y), magenta (M), and cyan (C) via an intermediate transfer belt serving as an image carrier, for forming a primary transfer nipping area. A transfer bias voltage having a polarity opposite to that of the toner images formed on the photosensitive drums is applied to the primary transfer rollers, thereby primarily transferring the toner images to the intermediate transfer belt.
A secondary transfer roller is disposed in facing the intermediate transfer belt to form a secondary transfer nipping area. A transfer bias voltage having a polarity opposite to that of the toner images on the intermediate transfer belt is applied to the secondary transfer roller, thereby transferring the toner images to a recording medium conveyed to the secondary transfer nipping area.
To obtain good secondary transfer consequences, it is required to control the transfer bias voltage applied to the secondary transfer roller to be in a suitable range. With such a prior art image forming apparatus, however, there arises a problem that the bias voltage cannot be suitably controlled in a case where the image forming apparatus makes printing on recording media in plural sizes and where the media have different widths in a direction perpendicular to a medium conveyance direction.